Frequency modulation receiver



P 1942- M. G. CROSBY 2,296,090

FREQUENCY MODULATION RECEIVER Flled April 6, 1940 3 Sheets-Sheet 1 INVENTOR ATTORNEY MURRAY G. CROSBY HETERODYNE RECEIVER SUPER- Sept. 15, 1942. M. s. CROSBY FREQUENCY MODULATION RECEIVER s Sh'ets-Sheet 2 Filed April 6, 1940 M 8 mm ma m w I .2 m w! m Y M B -Lm v. u n C m. n w u m Q u M n n m n u EEEQSY 7 E2 ww mq o Sept. 15, 1942. M. G. CROSBY FREQUENCY MODULATION RECEIVER 3 Sheets-Sheet '5 Filed April 6, 1940 INVENTOR MURRA Y G. CROSBY ATTQRN EY Patented Sept. 15, 1942 FREQUENCY MODULATION RECEIVER Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 6, 1940, Serial No. 328,353

4 Claims.

This application concerns new and improved detectors of the type in which the frequency modulated energy is passed through a wave phase or frequency retarding circuit and is caused to co-act with frequency modulated wave energy unretarded. Various circuits incorporating bandpass filters for the retard circuits are shown. Multi-grid detectors are used to combine the retarded and unretarded energy and produce a detected output.

In the prior art of frequency modulation detection by combining modulated waves with modulated waves passed by a retarding circuit as described in United States application #618,154, filed June 20, 1932, artificial lines have been used for the retard circuits and square-law detectors for the detector tube. In this application, I make use of band-pass filters to retard the modulated wave and the retarded voltage and unretarded voltage are applied to multi-grid detectors. That is, in this application the principle disclosed in my United States Patent #2,08'7,429, dated July 20, 1937, is utilized.

In describing my invention reference will be made to the attached drawings wherein:

Figs. 1, 2, 3 and 4 each show different forms of my frequency modulated wave demodulator comprising the Inulti-grid detector to the grids of which the voltage to be demodulated is fed over a direct path and a phase retarding path.

Fig. 3a illustrates a modification of the arrangementof Fig. 3.

Fig. 5 illustrates graphically the phase and amplitude characteristics of the band-pass filters or retard circuits plotted against frequency.

Fig. 6 is a vector diagram used to describe the operation of the detectors.

Figs. 7 and 8 are characteristic curves for the grids of the multi-grid detector tubes.

In the circuit of Fig. 1, the retardcircuit takes the form of transformer 2 having a tuned primary 4 coupled to a tuned secondary winding 6 which feeds one of the grids, say grid G1, of multi-grid detector 8. The modulated wave input to this retard circuit may be derived in any manner such as for example from a radio-frequency amplifier or as shown from the intermediate-frequency amplifier of a superheterodyne detector and amplifier illustrated diagrammatically at I. As stated above, voltage at the intermediate frequency is furnished by way of the retard circuit from superheterodyne receiver l to the grid G1 and also is supplied over a more direct path to another grid G2 of the detector 8 via coupling condensers I2 and I2 and switch S. Resistors i5 and I l properly damp the tuned input and tuned output windings of the transformer. By virtue of the multi-grid detector action as described more completely in my United States Patent #2,087,429 dated July 20, 1937, the

frequency modulated energy appearing at intermediate frequency at the output of the receiver l is detected so that the signal waves appear across resistor 24. These signalling Waves are made available for utilization at jack 24 by means of amplifier tube 26.

In the circuit of Fig. 2, the same principles of Fig. 1 are applied with a slightly different band-pass filter or retard circuit. In Fig. 2 the band-pass filter consists of tuned circuits and 32 which are coupled capacitively by condenser 34. The input and output of this filter are properly damped by resistors 36 and 38. Coupling and amplifying tube receives the frequency modulated energy upon its grid from potentiometer 4-3. Terminals X and Y are connected to the intermediate-frequency output of receiver I of Fig. 1. One of the grids, say G1, of multigrid detector 8 is fed by the output of the bandpass filter, comprising tuned circuits 3!] and 32 and the other grid, say G2, is fed by unretarded intermediate-frequency voltages via coupling condenser 12. Consequently, retarded and unretarded frequency modulalted energy will be present on the two grids so that the signalling Waves will appear across resistor 20 as in Fig. 1. Amplifier tube 26 makes the output available for utilization at jack 24. In this circuit the bandpass filter is shown capacitively coupled instead of inductively coupled as in Fig. 1. It will be apparent to those skilled in the art that condenser 34 of Fig. 2 may be replaced by an inductance and a blocking condenser or a resistance and a blocking condenser.

The circuit of Fig. 3 utilizes a retard circuit employing tuned circuits 40 and 42 which are coupled together by means of tuned circuit 44. Intermediate-frequency energy is applied to transformer 45 and amplified by coupling and amplifier tube 48. Tuned circuits 4%] and 42 are damped by resistances 4| and 43, respectively. The retarded voltage is supplied through the retard path to condenser 41 and from condenser 41 to the grid G1 of tube 8. The unretarded voltage is supplied over a more direct path including condenser l2 to the grid G2. Detection takes place here as in the prior modifications as will be described below and the detected output is made available for utilization in the plate circuit of multi-grid detector 8 by means of jack 50.

Fig. 3a shows an alternative connection of the band-pass filter or retard circuit. The connections of Fig. 3 are broken at X1, X2, and X3 and the circuit elements of Fig. 3a between the said points of X1, X2 and X3 damping resistors 4| and 43' are connected across the ends of the phase retarding network. In Fig. 3a, two series parallel-tuned circuits 54 and 56 are connected between the anode of tube 48 and coupling condenser 41' which corresponds to condenser 41 of bypass condenser 62.

1firstgrid G1 of the detector '8.

Fig. 3. The circuit 52 is connected in shunt to the series connections as shown and tuned to the mean intermediate frequency.

In the circuits of Figs. 1, 2, and 3 and 3a, cathode bias is shown for the detector tubes. This bias is produced in the condenser resistance units GR in the cathode circuits. This cathode bias may be utilized for either grid of the detector and a differential amount of bias may be applied to the other 'grid to take care of thedifferent biases which might be required on the two grids. Consequently, the first grid bias may be plus or minus depending upon whether its normal operating point is more or less negative than the operating point of the second grid.

In'the circuit of Fig. 4 the multi-grid detector Bis connected so asto produce a detector of the so-called infinite'impedance type. With this type of detection the audio output from the,

multi-grid detector-8 is" taken from its cathode resistor 60 shunted by intermediate-frequency The intermediate-frequency. energy is applied to terminals X and Y,

amplified by, tube 40 andfed directly to the second grid, of the-multi-grid G2 by way of condenser I2. ,The same energy is retarded by means of tunedtra-nsformer l4, l6 and fed to the Note these circuits; are quite similarto those shown in Fig. 1 of;the'drawings. Difierences in bias of the two grids Gi and G2 is taken care'of by means of -direct current source'8il which feeds bias voltage to the second grid Gz-through grid leak 32. How- .ever, by proper design of tube, this battery may be eliminated; The detected output is amplified by, tube Hand made available'for utilization at Allof thereceivers shown in Figs. 1, 2, 3, 3a and' l have retard'circuits consisting of bandpass filtershaving amplitude and phase char- 'acteristics as shown in Fig. 5. For further in- "formation in that respect, reference is made to '1 the"El,ectricalEngineers Handbook, by Pender and-McIlwain, volume V, section 7, pages 49 to *50. It will be noted thatthe phase characteristic is such that there is a permanent shift of 90 degrees for the carrier frequency and that the phase varies above and below 90 degrees as the fre-' quency is shifted-aboveand below the carrier frequency. Thus, the vector diagram of the primary and Es" with the phase The operation of the system disclosed here which causes demodulation of the frequency modulations to take place when one voltage is displaced 90 degrees and has a superimposed phase shiftwith frequency will now be described.

*The pentode detector 8 has a voltage'fed to one; grid,G1, which'has aphase given by :wherejFd is amount thefrequency is deviated with modulation from the carrier frequency. K is a constant which relates the phase shift due to'off-tuning in the--tunedcircuit to the frequency deviation from the inetune position.

of the-frequency modulatedwave, Fa varies in --accordance with-the-modulation so that it has In the case the value Fa sin pt where p=21r times the modulation frequency, Fm. Hence,

=g+KF sin pt (2) When the frequency modulated wave given by:

e =E cos (wt-F /F cos pt) (3) where w=21r timesthe carrier frequency, Fe, is applied as E inFig. 1, the drop across tuned circuit 2 willhave the phase given by (2) added toit or:

e E cos (wtF /F cos pt+1r/2+ KF sin pt) (4) The grids of the detector 8 are adjusted according'to the characteristics of Figures '7 and 8.

Both grids are adjusted to point a which is the linear portion of the characteristic. Thus, in accordance with the general detailed descrip- Ltionof this detector given in my United. States application Serial No. 716,469, filed March 20, 1934,Patent-#2,063,588, datedDecember 8, 1936,

- the variable current in the output of the detector would be substantially given by:

where Oil-811C102 are constants of the tube and e1 andez; are thetwo grid voltages. Applying the unshifted voltage of Equation 3 to one grid, say G2, and the phase shifted voltage of (4) to the other grid, say .61, gives 'as the variable output:

Simplifying and eliminating the radio-frequency .terms which would be eliminated by the lowpass filter in the detector output, the variable output would be:

By applying theBesselv function expansion:

sin (X sin ).=2J (X) sin +2J (X) sin 3| (10) the following is obtained:

J=a a E E [J (KF, sin pt+J (KF, sin 3pt+ .(11)

Thus, the output of detector 8 consists of the fundamentalfrequency, sin pt, proportional to a first order Bessel function of (KFd), and all the odd harmonics of the modulation frequency. The even harmonics are absent in the output of this detector. The frequency modulations on the wave havebeen demodulated and appear in the output circuit 20 of detector IE This method of demodulation has been described in Crosby application #25231, filed June 6, 1935, Patent No. 2,087,429, July 20,1937.

If the waves fed to the two grids G1 and G2 are amplitude modulated instead of frequency modulated, the two waves fed to the grids will be:

' e =E (l+ m, sin pt)'cos wt (12) V e =E (1+m sin pt) cos (wt+1r/2) (13) .where m is the amplitude modulation factor.

Substituting these in (5) gives:

This gives a radio-frequency term and a detected output given by:

2 (l-l-2m sin pH-m sin pt) The desired fundamental output is indicated by the term 2m sin pt in (1'7). It will also be noted that there is a second harmonic component m /Z sin 2 pt. This shows that amplitude modulation may be received if the carrier phase shift is made zero instead of 90 degrees. This phase shift may be obtained by making blocking condenser l2 and I2 small so as to have a high reactance in comparison to gridreturn resistor 82; for frequency detection I2 is small in reactance compared to the resistance of 82. The value of these blocking condensers is reduced by opening switch S to remove condenser l2' from the circuit. It is apparent that the optimum condition for amplitude modulation reception is with a phase shift of zero (or 180, or 360, etc.) degrees and the optimum condition for frequency modulation reception is with a phase shift of 90 (or 270, etc.). Thus it can be seen that if a frequency modulated wave is being received which is also amplitude modulated, the amplitude modulation is rejected in the absence of frequency modulating, during which time the phase is 90 degrees but as frequency modulation is applied, the phase departs towards zero and 180 degrees to let amplitude modulation through as the frequency deviates from the carrier frequency.

The above description principally concerns the type of detectors shown in Figs. 1, 2, and 3 in which the output is taken directly from the plate circuit of the tube. In the circuit of Fig. 4, the output is taken from the cathode circuit of the tube. However, since the cathode circuit is common to the plate circuit, it will be apparent that the operational characteristics are similar and need no further description.

There are two tunings which may be used in the circuits of Figs. 3 and 3a. In one the shuntconnected tuned circuit, or circuits, are tuned to the lower cut-off frequency of the band-pass filter and the series-connected tuned circuit, or circuits, are tuned to a higher frequency. In the other tuning, the shunt-connected tuned circuit, or circuits, are tuned to the upper cut-off frequency of the filter and the series-connected tuned circuit, or circuits, are tuned to a lower frequency. Either of these tunings work equally well since one effects a positive 90 degree phase shift at the carrier frequency and the other effects a negative shift.

What is claimed is:

1. In a system for demodulating a frequency modulated wave or an amplitude modulated wave,

an electron discharge tube having an output electrode, a first control electrode, a second control electrode and a cathode, a condenser for impressing either amplitude modulated wave energy or frequency modulated wave energy substantially directly between one of said control electrodes and cathode, a circuit parallel resonant substantially at the mean frequency of said amplitude modulated wave or frequency modulated wave energy a connection between spaced points on said parallel resonant circuit and the other of said control electrodes and cathode, means for adjusting the condenser to have a large reactance to demodulate said amplitude modulated wave and to have a small reactance to demodulate a frequency modulated wave, an output circuit connected with said cathode and said output circuit including a load resistor connected between the cathode and a point of relatively fixed alternating potential.

2. A system as recited in claim 1 wherein said parallel resonant circuit is shunted by an impedance to broaden its resonance characteristic.

3. In a system for demodulating amplitude modulated or frequency modulated wave energy, an electron discharge tube having an output electrode, a cathode and a plurality of control grid electrodes, a resistive load connected between the cathode and ground, a plurality of reactive circuits one of which is excited by either amplitude modulated or frequency modulated wave energy, a coupling between said reactive circuits, a coupling between one of said reactive circuits and one of said grid electrodes and ground, a reactance means for applying either amplitude modulated or frequency modulated wave energy substantially directly to the other of said grid electrodes, means for adjusting said reactance means to have a high reactance value for demodulating amplitude modulated Waves and to have a low reactance value for demodulating frequency modulated Waves, and a modulation frequency voltage output circuit connected across said resistive load.

4. In a system for demodulating amplitude modulated or frequency modulated wave energy, an electron discharge tube having an output electrode, a cathode and a plurality of control grid electrodes, a plurality of parallel resonant circuits one of which is excited by said either of said amplitude modulated or frequency modulated wave energy, said resonant circuits being tuned to the mean frequency of applied waves and there being sufficient coupling between said parallel resonant circuits to impart a band pass response characteristic thereto, a coupling between one of said parallel resonant circuits and one of said grid electrodes and said cathode, condensive means for impressing either amplitude modulated or frequency modulated wave energy substantially directly on the other of said grid electrodes and said cathode, a resistor connected from said other grid electrode to a point of invariable potential, means to adjust the condensive means to have either a large reactance relative to said resistor for amplitude modulation demodulation or a small reactance for demodulation of frequency modulated waves, and a modulation voltage output impedance in the space current path of said tube.

MURRAY G. CROSBY. 

